The present technology relates to non-volatile memory.
Semiconductor memory has become increasingly popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrically Erasable Programmable Read Only Memory (EEPROM) and flash memory are among the most popular non-volatile semiconductor memories. With flash memory, also a type of EEPROM, the contents of the whole memory array, or of a portion of the memory, can be erased in one step, in contrast to the traditional, full-featured EEPROM.
Both the traditional EEPROM and the flash memory utilize a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate. The floating gate is positioned between the source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage (Vth) of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned on to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
During a sensing operation, a voltage is applied to the control gate of a memory cell while a determination is made as to whether the memory cell is in a conductive state. If the memory cell is in a conductive state, its threshold voltage is less than the control gate voltage. On the other hand, if the memory cell is in a non-conductive state, its threshold voltage is greater than the control gate voltage. However, it can be problematic to provide a sufficiently low control gate voltage for sensing operations which occur during erase-verify, and during program-verify or read operations for lower programmed states.